The method and device according to the present invention use a technique that is well known in the art to identify defects that are visible on the surface of a substrate. It is a question of reflected darkfield microscopy, the principle of which is schematically shown by way of example in FIG. 1.
This technique consists in projecting an incident light ray 1 onto the surface of a substrate 4, for example, obliquely, at an angle β with respect to a plane P, parallel to the surface of the substrate to be observed. According to prior-art embodiments, it is also possible to project the light ray 1 perpendicularly to the surface of a substrate 4 (also called normal mode). The incident light ray 1 is thus directed in the direction of the surface of the substrate 4, for example using planar and/or concave mirrors 2, 3 allowing it to be concentrated on the surface of the substrate 4. Therefore, if the surface of the substrate to be observed were a defect-free planar mirror, the incident light ray 1 would be entirely reflected by the surface of the substrate 4 at the same angle β (called the “β” reflected ray and referenced by the reference 1′ in FIG. 1). Thus in this case, since the incident light ray 1 is not deviated, no light is scattered in the direction of a collecting channel 5 at the end of which a detecting unit 6 (such as a photomultiplier) is located; the latter detects the light intensity of a light ray scattered (i.e., reflected off the path of the “β” reflected ray 1′) by the surface of the substrate 4. In such a case, the detecting device will capture a uniform dark image.
In the case where the surface of the substrate 4 to be observed includes defects, some of the incident light ray 1 illuminating the surface of the substrate 4 is scattered by the defects in the direction of the collecting channel 5. The detecting unit 6 thus captures the light intensity thereof, which is converted into digital data, then transmitted to a data-processing means in order to be displayed, for example, on a screen 7. The obtained image is a representation in which the defects located on the surface of the substrate 4 appear light on a dark background.
It will be recalled that reflected darkfield illumination is particularly recommended for the study of surfaces. Reflected darkfield microscopy allows the amount of directly transmitted light to be minimized and only light deviated or scattered by defects located on the surface of the substrate 4 to be collected. It thus allows the contrast of the image illustrating the defects to be considerably increased while requiring relatively little equipment and simple preparation of the substrate 4. However, this technique suffers from the low light intensity collected and is always affected by a resolution limit.
An important field of application of this type of technology is the field of microelectronics. Specifically, in the semiconductor industry, reflected darkfield microscopy is used to inspect the surfaces of substrates, especially in order to detect particulates generated by various sources of contamination. Constantly progressing, this industry requires increasingly high product quality levels. By virtue of darkfield illumination, which is used in many pieces of metrology equipment, it is possible to detect particulates of size smaller than 0.1 microns, especially on silicon substrates.
Fully depleted semiconductor-on-insulator (FDSOI) (e.g., FD silicon-on-insulator) structures are increasingly used as substrates for the fabrication of components. In addition to surface particulates, other types of defects may be located in the silicon top layer forming the useful layer of the SOI structure; void-type defects, i.e., defects corresponding to zones devoid of the useful top layer, may especially be present in the top layer. To guarantee the quality level of SOI structures, it is essential to be able to identify and classify defects of this type that are smaller than 500 microns in size (defects of size larger than 500 microns being identifiable by other visual inspection techniques). Furthermore, since the required quality level continues to increase, the classification of defects smaller than 250 microns diameter, or even 100 microns diameter, may even be necessary in the near future. These defects, which are specific to SOI structures, have a signature in terms of the scattered light ray that is different from that of particulates.
The document US2004/0235206 discloses apparatus and methods for specimen inspection, to be applied to a bare substrate or a film stack deposited on a substrate. The method enables robust separation between signals of interest (for defects detection) and noise. Nevertheless, it does not allow the classification of the size of specific void-type defect.
Generally speaking, prior-art solutions do not allow void-type defects to be classed by size. Current measurements, which are obtained by virtue of pieces of metrology equipment intended to measure and count particulates, yield very imprecise classification results, thereby preventing SOI structures from being reliably sorted by the size of these “void” defects, to quantify their quality level.